State the two poles of a magnet. Describe what happens when like poles are brought near each other.
North and south poles. Like poles repel; unlike poles attract.
Magnetism & Electromagnetism
8 exam-style questions on magnetic fields, motors, generators and transformers. Tap Show answer for full working.
3 Easy3 Medium2 Hard
Describe the magnetic field pattern around a bar magnet. State the direction of the field lines.
Field lines emerge from the north pole and enter the south pole. They are closest together (strongest field) near the poles.
State Fleming’s left-hand rule and what each finger represents.
First finger: Field (N to S). Second finger: Current. Thumb: Motion (force). Used for the motor effect.
A wire carrying a current is placed in a magnetic field. State three factors that affect the size of the force on the wire.
Current size; magnetic field strength; length of wire in the field (and angle between current and field).
Explain how a simple d.c. motor works. Include the role of the split-ring commutator.
A current-carrying coil in a magnetic field experiences forces that produce a turning moment. The split-ring commutator reverses the current in the coil every half turn so the forces continue to rotate the coil in the same direction.
A transformer has 200 turns on the primary and 1000 turns on the secondary. The primary voltage is \(12\,\mathrm{V}\). Calculate the secondary voltage. Is this a step-up or step-down transformer?
\(V_s/V_p = N_s/N_p \Rightarrow V_s = 12\times1000/200 = 60\,\mathrm{V}\). Step-up.
Explain how an alternating current generator produces a voltage. Why does the voltage reverse direction twice each cycle?
A coil rotates in a magnetic field; the magnetic flux through the coil changes, inducing an EMF (Faraday’s law). After half a turn the flux change reverses direction relative to the coil orientation, so the induced EMF (and current) reverse direction twice per full rotation.
A step-down transformer is used to supply \(12\,\mathrm{V}\) from the \(230\,\mathrm{V}\) mains. The secondary current is \(5.0\,\mathrm{A}\). Assuming the transformer is \(100\%\) efficient, calculate the primary current.
\(V_p I_p = V_s I_s \Rightarrow I_p = (12\times5.0)/230 = 0.26\,\mathrm{A}\).